Subject: [PATCH v2 2/2] diffcore_rename(): use a stable sort

From: "Johannes Schindelin via GitGitGadget" <[email-masked]>
Date: Mon, 30 Sep 2019 10:21:55 -0700 (PDT)

From: Johannes Schindelin <[email-masked]>

During Git's rename detection, the file names are sorted. At the moment,
this job is performed by `qsort()`. As that function is not guaranteed
to implement a stable sort algorithm, this can lead to inconsistent
and/or surprising behavior: a rename might be detected differently
depending on the platform where Git was run.

The `qsort()` in MS Visual C's runtime does _not_ implement a stable
sort algorithm, and it even leads to an inconsistency leading to a test
failure in t3030.35 "merge-recursive remembers the names of all base
trees": a different code path than on Linux is taken in the rename
detection of an ambiguous rename between either `e` to `a` or
`a~Temporary merge branch 2_0` to `a` during a recursive merge,
unexpectedly resulting in a clean merge.

Let's use the stable sort provided by `git_stable_qsort()` to avoid this
inconsistency.

Signed-off-by: Johannes Schindelin <[email-masked]>
---
 diffcore-rename.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/diffcore-rename.c b/diffcore-rename.c
index 9624864858..9042936aba 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -585,7 +585,7 @@ void diffcore_rename(struct diff_options *options)
 	stop_progress(&progress);
 
 	/* cost matrix sorted by most to least similar pair */
-	QSORT(mx, dst_cnt * NUM_CANDIDATE_PER_DST, score_compare);
+	STABLE_QSORT(mx, dst_cnt * NUM_CANDIDATE_PER_DST, score_compare);
 
 	rename_count += find_renames(mx, dst_cnt, minimum_score, 0);
 	if (detect_rename == DIFF_DETECT_COPY)
-- 
gitgitgadget